The present invention relates to a method and apparatus for surface treatment of a roll, and more particularly, to a method and apparatus for surface treatment of a casting roll, by which a particular pattern may be quickly and accurately formed on a surface of the casting roll through etching.
In a typical twin-roll strip casting process, the shafts of a pair of rolls 1a and 1b are rotated in such a direction that the rolls are engaged with each other, and molten steel 2 is drawn through a nip which is formed between the rolls to thereby continuously cast a cast strip 4.
An apparatus which is used in such a process is called a twin-roll strip casting apparatus which is configured to manufacture the cast strip 4 by supplying molten steel 2 into a space between the pair of rolls 1a and 1b through a nozzle 3, and rapidly solidifying the molten steel 2 by an outflow of heat to the interior of the casting roll through contact between the rolls 1a and 1b and the molten steel 2 while rolling the rolls 1a and 1b. 
Here, an edge dam (not shown) made of a refractory material may be brought into close contact with both ends of each of the rolls 1a and 1b to prevent leakage of the molten steel 2. However, the edge dam may be easily worn through repeated contact and friction with the rolls 1a and 1b which are continuously rotating at a rapid rate.
Dust, such as wear debris, is produced at the edge dam (not shown) which is being worn out. Since introduction of such wear debris into the cast strip may cause defects such as cracks on the cast strip surface, a particular pattern may be formed on the surface of the rolls through etching to prevent the wear debris from being introduced into the cast strip.
When a particular pattern is formed on the surface of the roll through etching, it is possible not only to prevent the introduction of wear debris but also to effectively distribute the thermal stress in the cast strip 4. The cast strip 4 obtained is extremely thin at about 2 mm to about 6 mm, and thus a solidified shell which is formed initially has a large effect on the surface properties of the cast strip. However, since the contact time between the molten steel 2 and the rolls 1a and 1b is extremely short, the thickness of the solidified shell becomes non-uniform, and thus defects such as cracks or folding defects are formed on the surface of the cast strip. In this case, a subsequent rolling operation is negatively affected, and the surface quality of a final product may be degraded so that the value as a product declines.
Therefore, to prevent such defects, a method of suppressing the occurrence of surface defects by distributing the thermal stress in the cast strip 4 is used, and to this end, surface treatment by which dimples are formed on the surfaces of the rolls 1a and 1b is required.
Such dimples are formed by using the principle of making the thickness of the solidified shell become uniform by forming a gas gap as an insulating layer between the roll and the solidified shell to reduce the amount of heat which is drawn into the cooling roll, thereby inducing the solidified shell to be cooled slowly and also causing solidification to begin from the periphery of the dimple (rim portion of the dimple (recess)).
FIG. 2 illustrates an example of forming dimples on the surface of the roll in sequence. First, foreign substances adhered to the surface are removed to apply a masking film on the roll 1a. The applied masking film forms a coating which protects the surface of the roll 1a from an etching solution which is to be sprayed in a subsequent process.
After such a pretreatment has been completed, a photocurable photoresist resin or an acid resistant masking paint, which is in a liquid phase and curable at room temperature, is uniformly applied as a masking coating material to the surface of the roll through a spraying unit (not shown), and then dried at room temperature.
Thereafter, when the masking film has been formed on the surface of the roll, a laser unit 5 is used to remove only a portion of the masking film which is to be etched. Then, an etching solution is sprayed through an etching unit 8 onto the roll surface which has been patterned by the laser unit to thereby etch portions from which the masking film is removed. Consequently, fine dimples are formed, or the fine dimples contact each other to form a line or a plane.
Afterwards, the film is removed by a film removal unit (not shown) for removing the masking film which remains on the etched surface of the roll, and a finishing operation of washing the surface is performed, thereby completing formation of the dimples on the surface of the roll.
However, in the method described above, since the roll continually rotates at about 0.1 rpm to about 50 rpm, the maximum available time for a laser process is about 3 seconds, and is usually constrained to be less than about 3 seconds. Moreover, random patterns which follow a direction along the width of the roll cannot be connected, and thus there comes to be a limit on possible forms of the dimples which can be formed on the roll surface.
Moreover, when a vision system is used to realize precision in formation by a laser, the rolls must be stopped, which causes the operating time to be unnecessarily increased. Such an excess in operating time makes it difficult to optimize a patterning operation for each steel type, and thus there is no choice but to perform fabrication of a simple pattern, in which a simple shape is repeated, in order to reduce the operating time.
The simple pattern is shaped in such a way that a simple figure is repeatedly formed or a thin line is repeatedly formed with the objective of gas discharge. Since initial solidification conditions cannot be controlled through such a simple pattern, numerous strip fractures initially occur. In addition, even if casting continues, there is a limitation of being unable to prevent an adhesive phenomena, dent-type defect phenomena, etc., from occurring on the product surface.